1. Field of the Invention
The present invention relates to the field of semiconductor wafer processing and, more particularly, to performing end-point detection on a linear planarization tool used to planarize semiconductor wafers.
2. Background of the Related Art
The manufacture of an integrated circuit (IC) device requires the formation of various layers above a base semiconductor substrate, in order to form embedded structures over or in previous layers formed on the substrate. During the manufacturing process, certain portions of these layers need complete or partial removal to achieve the desired device structure. With diminishing feature size, such structures result in highly irregular surface topography causing manufacturing problems in the formation of thin film layers. To facilitate manufacturing processes, the rough surface topography has to be smoothened or planarized.
One of the methods for achieving planarization of the surface is chemical mechanical polishing (CMP). CMP is being extensively pursued to planarize a surface of a semiconductor wafer, such as a silicon wafer, at various stages of integrated circuit processing. CMP is also used in flattening optical surfaces, metrology samples, and various metal and semiconductor based substrates.
CMP is a technique in which a chemical slurry is used along with a polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer, in combination with the chemical reaction of the slurry disposed between the wafer and the pad, provide the abrasive force with chemical erosion to planarize the exposed surface of the wafer (typically, a layer formed on the wafer), when subjected to a force pressing the wafer onto the pad. In the most common method of performing CMP, a substrate is mounted on a polishing head which rotates against a polishing pad placed on a rotating table (see, for example, U.S. Pat. No. 5,329,732). The mechanical force for polishing is derived from the rotating table speed and the downward force on the head. The chemical slurry is constantly transferred under the polishing head. Rotation of the polishing head helps in the slurry delivery, as well as in averaging the polishing rates across the substrate surface.
Another technique for performing CMP to obtain a more effective polishing rate is using the linear planarization technology. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated for averaging out the local variations, but the planarization uniformity is improved over CMP tools using rotating pads, partly due to the elimination of unequal radial velocities. One such example of a linear polisher is described in U.S. Pat. No. 5,692,947.
Unlike the hardened table top of a rotating polisher, linear planarizing tools are capable of using linearly moving belts upon which the pad is disposed. The ability for the belt to flex can cause a change in the pad pressure being exerted on the wafer. When the pressure of the wafer-pad engagement can be controlled, it provides a mechanism for adjusting the planarization rate and/or the polishing profile across the surface of the wafer. Therefore, a fluid support (or platen) can be placed under the belt for use in adjusting the pad pressure being exerted on the wafer. An example of a fluid support is disclosed in U.S. Pat. No. 5,558,568.
When CMP is employed, it is generally advantageous to monitor the effects of the planarizing process to determine if the process is being performed according to desired specifications. A monitoring problem specific to CMP is the determination of the process end point. That is, the ability to monitor the material thickness being removed and to terminate the polishing when a certain end point condition is reached. A typical end point is the case when one material is removed to exposed an underlying material, which is different from the first material. An end point detection technique detects this point where the CMP process is to be stopped.
Various schemes have been devised to detect an end point during CMP. For example, one technique relies on conductivity measurements (see for example, U.S. Pat. Nos. 4,793,895 and 5,321,304). Another technique employs the monitoring of the electrical current to a motor which rotates the wafer (see for example, U.S. Pat. No. 5,308,438). Still another technique uses an acoustic wave reflection to monitor dielectric thickness (see for example, U.S. Pat. No. 5,240,552). Optical techniques are now being implemented as an accurate indicator for measuring material thickness on a wafer (see for example, U.S. Pat. No. 5,433,651). Additionally, Chingfu Lin et al. have demonstrated the use of pad temperature as a method for the determination of polish end point (see, "Pad Temperature As An End Point Detection Method in WCMP Process;" 1998 CMP-MIC Conference; Feb. 19-20, 1998; pp. 52-56). Accordingly, it is understood that a number of techniques are available for detecting the end point of a polishing cycle for a semiconductor wafer.
The historical approaches for in-situ monitoring of the end point pertains mainly to rotating (orbital) polishers. Linear polishing techniques allow for alternative techniques to be developed to take advantage of the linearly moving pad/belt of the linear planarization tools. The present invention implements an end point detection scheme for CMP, which relies on an operative phenomenon different from previous techniques, but is still simple in its approach. The present invention is operative with linear planarization tools, but can be readily adapted to other techniques, including rotating polishers.